[katieHaylor (OP)]

Stuart asks:

If you could bring a baby from the past to grow up in the present, how far back could you go before people would notice that this was a time travelling baby?

What do you think?

[Halc]

First of all, I like the fairly defined way the question is posed.  By letting the baby grow up in the present, you don't get the cultural shock of the caveman being drop-jawed at the technology.

So physically, I think you have to go a long way back.  There are some really weird looking people out there, and suggestions that the person might be a primitive version are not serious suggestions.  So I think other evidence would be more telling.

One: Go a moderate way back and the baby will not have the benefit of the last say million years of the perpetual evolution between us and the stuff trying to kill us.  The baby would have a hard time growing up at all since all these diseases that have bred to prey on humans will find the baby fairly defenseless, much in the way Native Americans lacked genetic defenses against European diseases like Measles and Syphilis, but Europeans were suddenly exposed to diseases brought back from the west.
We're going to have one sick baby, and doctors wondering about this might do a genetic test that will definitely make them notice the time travelling baby.

On the longer front, there becomes a point where our baby is no longer human, where the being can not breed with a modern human.  Between the extremes of human and totally not-human is the middle ground such as exists between horses and donkeys where they can breed, but the offspring is always infertile (a mule).

Notice that I don't offer any specific numbers as to how far back one has to go.  The 'million years' remark was just a number at random.  It's not like something changed at that point making subsequent humans more generally immune from stuff.  That's a continuous process.

[syhprum]

I have seen it suggested that as far as appearance goes Lucy would fit in with modern humans.

[Bored chemist]

There are as many answers as there are definitions of "modern day humans".
How about this one?
https://en.wikipedia.org/wiki/Capital_punishment_by_country

[dead cat]

There are as many answers as there are definitions of "modern day humans".
How about this one?
https://en.wikipedia.org/wiki/Capital_punishment_by_country

wiki is always good if you post the correct link I dont think capital puinishemnt has anything to do with the question on this thread. Perhaps you meant to post the following https://en.wikipedia.org/wiki/Homo

When you go far enough back into extinct species it is speculation re if we are related or not. I guess if an Australopithecus was raised in by humans, they might think they have a chimpanzee.

Edit: If any took the time to read the link, did you notice just how big Neanderthals brain is compared to the average human. Apparently most europeans and asians are typically 4% neanderthal. Aboriginals have a high amount of Denovian DNA. Conclusion : The older versions of humans have not completely died out yet. Neanderthals inter bread with Homo Sapiens that evolved in Africa resulting in smaller skulls, and easier child birth maybe.

[jimbobghost]

obviously, we are not there yet.

since modern humans have a smaller brain size than Neanderthals (and larger brain size was once considered an advancement in intelligence); modern humans have compensated for lack of brain size by advancing technology...which is a recent development.

since a smaller brain size has resulted in "modern humans"; consider the brain size of modern women to modern men.

women have smaller brains, yet have convinced men to support them; in exchange for sexual favors (that they actually require in order to reproduce)...and so receive "two bangs for the buck".

therefor, I submit that given the smaller brains that women possess, and the obvious gains the smaller brain has given them: that when the human brain has shrunk an additional 50 pct or so; modern humans will have reached it's full potential: brilliant women controlling male drones for women's final and total achievement.

[Bored chemist]

wiki is always good if you post the correct link  I dont think capital puinishemnt has anything to do with the question on this thread.

I assure you, I posted the right link.
Modern humans don't have a death penalty.
Only outdated theocracies  have that.

I'm not claiming that mine is the only definition of "modern humans", but, until you agree a definition, you can't answer the question.

[jimbobghost]

"Modern humans don't have a death penalty."

will you explain what you mean by that?

(i'm sure you can...was that meaning to suggest that those contries who support the death penalty are not those lead by "modern humans"?)

you also mentioned "outdated theocracies"...did you also mean to suggest that countries lead by religious beliefs are less modern that those of a more atheistic/agnostic culture?

[David Cooper]

You might notice the difference with a neanderthal or denisovan baby visually, but we don't know how well they'd do mentally in our society. Even if they're less bright, there's a good chance that they'd still be within our normal range. Going back to homo erectus would likely show up a greater deficit, and a much more visual difference, but something about the design of the homo erectus mind may have been very right, because it quickly led to us and our two closest cousins perhaps largely through a growth in brain capacity to take advantage of some key breakthrough with intelligence (and likely with language).

The way to test this though is to recreate them. Do we have enough of their DNA to do this?

[guest46746]

The way to test this though is to recreate them. Do we have enough of their DNA to do this?

What must be considered, is that Neanderthals lived in isolated pockets as their population shrunk, This undoubtedly lead to an interbreeding that deplenished their gene pool. This mostly lead to a swift takeover by modern humans. The Neanderthals that interbred with modern humans were probably less affected by interbreeding. The migrations of modern humans into Europe  was likely not possible  during a period of a genetically healthy Neanderthal population. lol   

[katieHaylor (OP)]

This question has now been answered, and you can listen to it here - https://www.thenakedscientists.com/podcasts/question-week/when-did-we-become-modern-day-humans

[Petrochemicals]

obviously, we are not there yet.

since modern humans have a smaller brain size than Neanderthals (and larger brain size was once considered an advancement in intelligence); modern humans have compensated for lack of brain size by advancing technology...which is a recent development.

since a smaller brain size has resulted in "modern humans"; consider the brain size of modern women to modern men.

women have smaller brains, yet have convinced men to support them; in exchange for sexual favors (that they actually require in order to reproduce)...and so receive "two bangs for the buck".

therefor, I submit that given the smaller brains that women possess, and the obvious gains the smaller brain has given them: that when the human brain has shrunk an additional 50 pct or so; modern humans will have reached it's full potential: brilliant women controlling male drones for women's final and total achievement.

Yup were not there yet, so tempted to say only if youve got the right jeans and have the correct facebook.

Ithe neanderthals where a red haired socially minded group apparently, who where pushed northwards. They did not have bigger brains when body mass is considered, ie the human brain is not the largest on the earth, nor the largest ratio to body tipe

https://en.m.wikipedia.org/wiki/Brain-to-body_mass_ratio
 
Neanderthals where better at lateral social thinking, but worse at frontal coretex problems such as disputes and attacks etc. Hypothetically any interbreeding would have produced a hybrid superior in both. Women are utterly devious still.

[Bored chemist]

"Modern humans don't have a death penalty."

will you explain what you mean by that?

(i'm sure you can...was that meaning to suggest that those contries who support the death penalty are not those lead by "modern humans"?)

you also mentioned "outdated theocracies"...did you also mean to suggest that countries lead by religious beliefs are less modern that those of a more atheistic/agnostic culture?

You have deduced my  opinions reasonably accurately.
I don't think much of theocracies or countries who haven't abolished the death penalty yet.
I should, perhaps, clarify that in this case I include "The Party" or "Money" among the deities that the theocracies might worship.

[set fair]

Not an answer to the OP but perhaps the big change happened when somebody discovered the law of supply and demand and armed with this found that improved technology, if the "how" were kept secret, could make you rich* by limitimg supply thus spurring invention.

* rich in possesions supposing the discovery predated money.